1. Technical Field
This invention relates to controlling the air-to-fuel ratio of a combustion process, in particular the combustion process in an internal combustion engine.
2. Prior Art
In recent years, high temperature solid state oxygen sensors have been used extensively for control of the air-to-fuel ratio, A/F, of internal combustion engines. The most widely used device is a simple Nernst concentration ZrO.sub.2 cell which gives an output EMF proportional to the natural logarithm of the oxygen partial pressure of the exhaust gas (P.sub.O, exh), ##EQU1## where P.sub.O,ref is the oxygen partial pressure of a reference atmosphere, e.g. air, F and R are the Faraday and the ideal gas constants, and T is the absolute temperature. Despite the sensor's low sensitivity to oxygen partial pressure, the large change in oxygen pressure at the stoichiometric A/F ratio allows the successful use of these sensors for stoichiometric A/F engine control. On the other hand, the usefulness of these sensors in the lean A/F region of the weak dependence of P.sub.O,exh on A/F in the lean region. Recently, a number of oxygen sensors with lean A/F measurement capabilities have been reported. These devices are commonly called lean exhaust gas oxygen, LEGO, sensors. The devices are based on oxygen pumping with ZrO.sub.2 cells and provide an output which generally is linearly proportional to the oxygen partial pressure. Their high sensitivity to oxygen pressure combined with low temperature sensitivity make these sensors promising for lean A/F engine control.
A double ZrO.sub.2 cell sensor described by Hetrick et al (U.S. Pat. Nos. 4,272,331, 4,272,330 and 4,272,329) includes two ZrO.sub.2 electrochemical cells, one used as an oxygen pumping cell and the other as an oxygen sensing cell. FIG. 1 shows schematically the structure of this sensor. The device is immersed in the exhaust gas and a current I.sub.p is passed through pumping cell 10 that pumps oxygen out of the cavity 11. This pumping action lowers the oxygen pressure inside cavity 11 and induces a diffusional flux of oxygen from the exhaust into the cavity 11 through the aperture 12 and, at the same time, causes an EMF V.sub.s to develop across a sensor cell 13. At steady state, the flux of oxygen pumped out of cavity 11 by the current is equal to the diffusional flux of oxygen into cavity 11 and the following relation between I.sub.p, V.sub.s and P.sub.O,exh is then valid: EQU I.sub.p =4e.sigma. P.sub.O,exh (1-exp(-4FV.sub.s /RT)) (1)
where e is the electron charge, F and R are the Faraday and the ideal gas constants, T is the absolute temperature and .sigma. is a constant that depends on the geometrical characteristics of the aperture 12 and the diffusion constant of oxygen.
During operation of the device, the current I.sub.p is adjusted so that the voltage V.sub.s is kept constant (e.g. 50 mV). This is accomplished with a feedback circuit which compares the EMF, V.sub.s, with a reference voltage V.sub.r. Under these conditions, I.sub.p is proportional to P.sub.O,exh.
FIG. 2 shows the dependence of the current output I.sub.p of this device on the air-to-fuel ratio of an engine. To control the A/F ratio at a desired (A/F).sub.c, an external feedback circuit is used which compares the sensor output V=I.sub.p R with a reference voltage V.sub.c =(I.sub.p).sub.c R corresponding to the desired (A/F).sub.c (see FIG. 1b). This comparison could be done in an on-board computer utilizing what is commonly called a "look-up table", a tabulation of V.sub.c vs A/F which is essentially a calibration curve for the device. If V is different than V.sub.c, the external feedback circuit adjusts the A/F ratio (e.g. by adjusting the amount of fuel) until V becomes equal to V.sub.c.
A number of single ZrO.sub.2 cell O.sub.2 pumping devices have also been described, in which the single ZrO.sub.2 cell acts as both an O.sub.2 pump and an O.sub.2 sensor. The "diffusion barrier" in these devices is formed either by a porous inactive layer (e.g. spinel) or by a small cavity with an aperture. If one applies a sufficiently high voltage across the ZrO.sub.2 cell, the pumping current attains a limiting value (saturation current) which is proportional to the partial pressure of O.sub.2 in the ambient (exhaust gas). Saturation represents the complete depletion of O.sub.2 at the negative electrode.
In addition to O.sub.2 pumping-based sensors intended for lean operation, devices have also been described which can operate over an extended A/F range from very lean to very rich A/F mixtures. These devices are commonly called universal exhaust gas oxygen, UEGO, sensors. One example of this type of sensors was discussed by Vassell et al in Society of Automotive Engineers Paper #841250.
Double-cell oxygen-pumping devices such as the one described by Hetrick et al have several advantages over the single-cell sensors. These include insensitivity to electrode properties, operation not limited by the resistance of the ZrO.sub.2 material and more effective optimization with respect to response time and temperature sensitivity. Double-cell sensors, on the other hand, are more complex in structure, electronics and operation. It would be desirable, therefore, to develop devices which preserve the essential advantages of the above-mentioned double-cell sensors but are simpler. These are some of the advantages of the present invention.